For certain applications such, for example, as sharpening drill bits it is necessary to hold the bit at a first angle during the initial grinding operation and then to hold the bit at a second angle during the subsequent grinding operation. A fluid operated rotary cylinder such as described in U.S. Pat. No. 3,131,610 may be connected to a collet for automatically rotating the drill bits between the first and second angular positions. My copending application Ser. No. 483,531 filed June 27, 1974, now U.S. Pat. No. 3,905,157 discloses a grinding machine incorporating such a rotary cylinder. In that machine, it is necessary for the operator to manually release the bit from the tool holder prior to removal and to lock the bit in the tool holder after insertion therein.
In order to increase the rate at which drill bits can be sharpened in the machine described in my said application it would be desirable to employ an automatic collet which automatically releases the bits at the end of each complete sharpening operation and which locks the bits in the collet at the beginning of each such operation. Not only does the automatic collet increase the effective speed of the machine but it adds an additional safety factor by assuring that each bit is firmly locked in the collet when engaged with the grinding wheel.